The present invention relates to a Pulse Width Modulation (PWM)/linear driver for an electromagnetic load, particularly a PWM/linear driver for a hard disk of the type Voice Coil Motor (VCM).
Contemporary hard disk drivers typically include a brushless DC motor within which a disk rotating spindle assembly for rotating at least one data storage disk at a desirable velocity, typically in a range between 5,000 and 10,000 revolutions per minute, is provided.
Hard disk drivers also typically include a rotary actuator structure powered by a Voice Coil Motor (VCM). The rotary actuator structure positions one or more slider head assemblies at desired locations relative to surfaces of the disk or disks.
A storage disk device such as a magnetic disk device etc. has been widely utilised as a storage device of a computer etc.
A high-density record and a decrease in the electric power consumed are demanded for this type of storage disk device. For reducing the electric power consumed by this magnetic disk device, a PWM driving system is applied to a driving stage of the VCM.
In U.S. Pat. No. 5,917,720 a method and a circuit for driving a bridge by a PWM procedure is described, wherein the current load is controlled by modifying the duty cycle of the two outputs of the PWM power stage.
In fact referring to the FIG. 1, the condition of no current is obtained by driving the two outputs OUTP and OUTM by means of two signals having the same frequency and the same duty cycle, that is equal to 50%.
By incrementing the duty cycle of the output OUTP and by decreasing the duty cycle of the output OUTM, or vice versa, the current will flow in the load with a direction and an intensity that depends on the difference of the duty cycles of the two outputs.
The current sensing is performed by a sensing resistance Rs placed in series to the load; the voltage generated at the ends of said sensing resistance is opportunely amplified by an operational amplifier, called Sense Amplifier, and said voltage is used as a feedback to close the current control loop of the circuit.
However, this circuit due to the switching frequency of the power stage PWM, of about 100 KHz, and due to the low common mode rejection of the Sense Amplifier at said frequencies, has the output signal of the Sense Amplifier disturbed by the fast common mode variation to which the inputs are subjected.
In U.S. Pat. No. 6,061,258 another embodiment for monitoring the current in an inductive load driven by a PWM power stage is described.
Also in this embodiment there are problems with the common mode rejection, even if the current sensing is made synchronous with the switching frequency of the PWM power stage.
In the heretofore cited patents, the basic concept was to drive the power stage by a PWM power amplifier.
To prevent these drawbacks, some systems able to drive the output stage both in linear and PWM modalities have been implemented.
In fact, in EP Patent No. 962915, a schematic PWM/linear driver circuit for a VCM according to the prior art is described.
As illustrated in FIG. 2, a magnetic disk apparatus includes a magnetic disk 1 and a magnetic head 2. Usually the magnetic disk 1 has a data surface on which servo signals are embedded in data tracks. The magnetic head 2 reads and writes information from and to the magnetic disk 1. An Integrated Circuit (IC) head 3 amplifies the signal read by the magnetic head 2. An IC read channel 4 selects the read signal from the head IC 3. A servo signal demodulator 5 demodulates the servo signal from the read channel IC 4 into a position signal.
A servo gate generation circuit (not shown in FIG. 2) generates a servo gate signal, synchronizing with rotations of the magnetic disk 1.
A control circuit 7 consists of a processor. The control circuit 7 reads a position signal in accordance with the servo gate signal. Then, the control circuit 7 detects a present position of the head 2 from the position signal, and generates a current indication value corresponding to a distance to a target position from the present position.
A Digital to Analog Converter (DAC) 8 converts the current indication value given from the control circuit 7 into an analog quantity.
A PWM driver 9 becomes operable when a PWM enable signal from the control circuit 7 is at a high level, and outputs driving current for PWM-driving a coil edge voltage in accordance with the current indication value given from the DAC converter 8.
A linear driver 10 operates and outputs a driving current of which a magnitude corresponds to the current indication value given from the DAC converter 8.
An AND gate 11 takes the logic function AND of the servo gate signal and the PWM enable signal, and generates a switchover signal SW.
A coil driving circuit 12 includes four power transistors Q1-Q4. A coil VCM of the actuator is connected to middle point of the four power transistors Q1-Q4. A current detection resistor RS detects an electric current flowing to the coil VCM.
A differential amplifier 13 generates an inter-terminal voltage of the current detection resistor RS, and feeds back this voltage to the PWM driver 9 and the linear driver 10 as well. An analog switch 14 selects the output of the PWM driver 9 or of the linear driver 10 in function of the switchover signal SW.
Further schematic PWM/linear driver circuits for a VCM according to the prior art, in by way of example, are described in U.S. Pat. Nos. 5,631,817 and 5,838,515.
All these patents have a basic concept, that is, during the seek control, also called coarse control, the VCM is PWMxe2x80x94driven and during the track operation, also called fine control, the VCM is linearly driven.
Such a technique of the output stage driving has the drawback of integrating the two pre-driver of said output stage, that is the linear and PWM drivers, with a consequent increment of the necessary silicon area. Moreover the transition between the linear modality and the PWM modality, and vice versa, is not executed instantaneously, but the circuit is perturbed by such a transition.
In view of the state of the art described, it is an object of the present invention making a circuit able to select which type of operative modality has to be chosen without any further circuits.
According to the present invention, such object is achieved by a bridge circuit of the type having a signal input and a signal output and at least two conduction control inputs for driving a voice coil motor in a linear mode and in a pulse width modulation (PWM), the bridge circuit characterized in that it is driven by a PWM converter coupled to one of said two control inputs and by a linear amplifier coupled to the other of said two control inputs.
Such object is also achieved by a method for driving a bridge circuit of the type having a signal input and a signal output and at least two conduction control inputs for driving a voice coil motor in a linear mode and in a pulse width modulation, characterized by comprising the step of driving simultaneously in a PWM procedure one of said two control inputs and in a linear procedure the other of said two control inputs.
Thanks to the present invention it is possible making a PWM/linear driver for an electromagnetic load without discontinuity in the control loop.
Moreover, thanks to the present invention it is possible making a PWM/linear driver for an electromagnetic load easier in its implementation with respect to the prior art embodiments.
Furthermore, thanks to the present invention it is possible making a PWM/linear driver that, due to the linear driver, does not hamper with the read and write signals of the HDD and, thanks to the PWM driver, consumes less power with respect to the known drivers.